Comment by autocorr

I think this is the main thing, which is that the quality of the discussion really depends on the topic. As a post-doc in astronomy, I'm happy to report that the quality of HN discussion on astro-related submissions tends to be quite good[1]. The top level posts tend to be either be broad-level questions that get some good replies or "fun facts". This probably has to do with most folks being interested in space but not really having strong opinions, leading to cordial comment sections and a decent supply of actual astronomy experts on HN.

Now, for things like finance, markets, politics, etc. like you're getting at, those are a different store. I've been baffled that some political and finance posts have devolved to threads worse than Reddit... Maybe some subs have become homogeneous enough that they don't generate quite the same conflict?

[1] About once a year there's a Dark Matter related post that usually brings out a very critical set of responses. That and Dark Energy are just things you need a lot of investigation into the evidence of because, like Quantum Mechanics, they are just weird and don't make sense to the uninitiated.